If you want 27" wheels don't half ass it, buy a 27" frame. They've been around long enough now where you can find them cheap. 27" wheels on 26" frame will jack up your BB, and reduce mud clearance to nothing. Because 650b is technically only 25mm taller you can run 650b in pretty much any 26" frame, but the negatives of doing so outweigh the .5" increased attack angle.